Current KE penetrators are long-rod projectiles (0.5-2 m) that are fired at high velocities, typically 1.6% m/s to 3.0 km/s by the use of a sabot. These “arrow-like” projectiles are machined from high mass-density materials, such as but not limited to depleted uranium (DU) alloys (18.5 g/cc) and tungsten (W) composites (17.5 g/cc). FIG. 1 is a picture of a typical KE penetrator wherein the sabot has begun to separate.
The performance of DU alloy KE penetrators is believed to be generally superior to comparable density W composite KE penetrators. This is attributed to the DU alloy's susceptibility to adiabatic shear (AS) localization and failure. Under these conditions, the heat generated by the high rate deformation causes thermal softening mechanisms within the penetrator material to compete and eventually overcome the material's work-hardening mechanisms. The plastic deformation can become unstable and the deformation can tend to focus into the plastic localizations known as AS bands. The shear bands provide a mechanism by which the DU penetrator can rapidly discard the deforming material at its head, preventing the build-up of the large “mushroomed” head observed on the W-alloy penetrators. This “self sharpening” behavior allows a DU penetrator to displace a narrower but deeper penetration tunnel, and thus, to burrow through armor protection more efficiently.
FIG. 2 depicts two different penetration mechanisms—FIG. 2A depicts adiabatic shear failure in DU resulting in ‘self-sharpening’; and FIG. 2b depicts work hardening causing mushrooming in tungsten heavy alloy armor (WHA). As seen in FIG. 2, the penetrator mushrooms within the target, with macroscopic plastic deformation followed by erosion. The initial strain is principally localized within the matrix, which rapidly work hardens to form the mushroom shape. A consequence of the mushrooming due to work hardening is that energy is expended radially to expand the penetration cavity. In DU, unlike in tungsten heavy alloy (WHA), the thermal softening overcomes the increase in flow stress, permitting adiabatic shearing to occur. This results in a ‘self-sharpening’ of the penetrator, as the mushroom head is continually sheared from the penetrator body, as seen in FIG. 2a. The net result is less energy expended in expanding the penetration cavity radially, with a concomitant increase in energy available for axial penetration.
Impacts against hard targets, e.g., rolled homogeneous armor, result in local temperatures as high as 2,500K and pressures at the penetrator/target interface of 5 to 10 GPa. This results in a phase change in uranium from solid to liquid. At these elevated temperatures, the uranium reacts readily with atmospheric oxygen. The oxides formed subsequently condense to solid aerosol particles. Oxidation is the source of the pyrophoric nature of DU impacts and is not present with WHA impacts. This burning effect provides an additional advantage effectiveness of DU penetrators, particularly inside the target. Much work has been conducted in the US on determining the extent to which penetrators are converted to aerosols and on characterizing the aerosol particle size distributions. Against hard targets, it is estimated that 18% of the DU penetrator of 120 mm tank munitions is aerosolized, with virtually all these aerosols (91 to 96%) having sizes <10 um.
Both the DU and WHA penetrators are effective at piercing through the armor; however there are environmental concerns associated with using the DU. This is being addressed by developing W-based composites with ballistic performances equaling or surpassing that of DU. The conventional W composites are produced by liquid-phase sintering elemental powders of tungsten, nickel, iron and/or cobalt to produce a two-phase composite of W particles (typically 30 μm to 50 μm in diameter) embedded in a nickel alloy matrix. The solid state processing technique of ball milling subjects a blend of powders to highly energetic compressive impact forces that produce alloy powders by repeated cold welding and fracturing of the powder particles has shown to give improvements. The ball milling, which is considered to be a far from equilibrium process (even more so than rapid solidification), yields not only nanograined powder (grain size <100 nm), but also alloys with extended solid solutions. These nanograined powders also may be consolidated at significantly lower temperatures than those used for liquid phase sintered W composites, avoiding the formation of undesirable phases. The high strengths of nanocrystalline metals and alloys, and the saturation or reduction of their work-hardening capacities, can make them prone to shear failure modes, which may mimic the DU rounds.
While new W-composites address the environmental issue, they do not address the issue of poor behind armor damage that is generally associated with KE penetrators. Most KE penetrators do not have any explosives because the high impact pressures and temperatures would cause the explosive to detonate. Additionally, if denotation occurs upon impact, the explosive force would work directly against the penetration force and reduce the amount of penetration. Also, the chemical energy of the explosive would be released in front of the armor and not behind the armor where it can do the most damage. Finally, the addition of conventional explosives which are typically 1-3 gm/cc would substantially lighten the KE penetrator and reduce its penetration effectiveness.
Some of these issues have been addressed by the following methods. One method to improve KE weapons is the PELE ammunition developed in cooperation with GEKE Technologies GmbH from Freiburg, Breisgau. This ammunition does not contain any explosives and is based using a two-component rod consisting of an outer shell and an inner core with different bulk modulus of compressibility and densities. The design works on the simple physical principle: when the penetrator strikes a target, the material in the core is compressed because of its lower density. This compression exerts a pressure on the inside of the shell which forces the warhead apart, producing a large number of fragments which can only move in the direction of firing. Consequently, the effect is limited to a confined and defined area. While this does help improve the behind armor damage, it still only provides kinetic energy and the amount of penetration is reduced.
Another method to enhance KE weapons is provided in U.S. Pat. No. 5,728,968, issued Mar. 17, 1998 to Buzzett, et al. (“the '968 patent”). Such '968 patent invention uses a typical KE round that contains a forward compartment and a rearward compartment separated by a small diameter passageway; all containing a pyrotechnic mixture. The pyrotechnic mixture is a thermite type material containing aluminum, iron oxide, nickel and a fluorocarbon binder. Upon impact the front cavity ignites due to the high temperature and pressure created upon impact. This in turn spontaneously ignites the rest of the pyrotechnic material. The confined space of the rearward compartment creates a high reaction temperature and pressure resulting in molten metal and metal oxide being jetted out the front of the projectile through the small diameter passageway. This chemical energy associated with the jet assists in penetration of the target and creating behind armor damage. In this invention of the '968 patent, the rear cavity and the small diameter bore are required to contain the thermite type material while it is reacting so that the pressure and temperature will build to a condition that material is propelled out the small diameter bore. This requires extensive machining and limits the amount of energetic material that can be carried to the target.
Hence there still exists a need to more efficiently couple a kinetic energy projectile to a target, produce more behind armor damage and be able to provide more chemical energy to assist in the behind armor damage.